This invention relates to testing and evaluating cathodic protection effectiveness on buried or submerged metallic structures, specifically to the evaluation of corrosion protection levels on coated pipelines, tanks, piles, and piping systems.
Buried or submerged metallic structures, such as pipelines, tanks, and distribution piping systems are usually coated with non-conductive material to prevent corrosion. If any corrosion occurs in any uncoated areas of the structure many adverse affects will occur, such as loss of transported product, possible fires and explosions caused by leaking gas and fuel oil, and contamination of the environment. To prevent these effects, most pipelines are provided with corrosion protection consisting of cathodic protection, in addition to the non-conductive coating.
Cathodic protection provides corrosion protection to any bare metal areas exposed to soil due to coating defects or "holidays" by causing direct current to flow from the soil into the structure, thereby polarizing the structure as a cathode. The required direct current output of the cathodic protection system is reduced to manageable levels by the coating, which substantially reduces the bare metal area of the structure exposed to soil.
Two cathodic protection systems are in use for corrosion protection of metal structures. The first, termed an impressed current cathodic protection system, consists of a rectifier, insulated wires connecting the plus terminal of the rectifier to a buried anode (for instance graphite cylinders), insulated wire connecting the negative terminal of the rectifier to the protected structure, and test stations installed at the structure. The test stations typically consist of a pipe or a valve box with one or two insulated wires attached to the structure, typically by brazing, and a terminal board for termination of the wires. The test stations are used for monitoring the corrosion protection levels by measuring potentials between the structure and a reference electrode in an electrical contact with ground above the structure. The reference electrode usually consists of a copper rod fixed in a plastic body filled with saturated copper-sulfate solution, and having a porous plug to facilitate electrical contact with the ground.
The second, termed a sacrificial (galvanic) cathodic protection system, consists of magnesium, zinc or aluminum anodes buried next to the structure and often directly connected by an insulated wire to the structure. The protective current is generated by the potential difference between the structure and the anode. The structure with sacrificial anodes also has test stations for the cathodic protection testing and evaluation of its corrosion protection effectiveness.
Details of different cathodic protection systems and of the pipeline potential measurements can be found in Metals Handbook, Volume 13, Corrosion, published by ASM International, Metals Park, Ohio, 1987.
The objective of the cathodic protection is to shift the potential of the structure in a negative direction. The potential shift must be large enough to mitigate structure corrosion. Potential criteria have been developed by the National Association of Corrosion Engineers (RP0169-92) to provide guidance for determination of safe cathodic protection levels to mitigate corrosion. One of the criteria is based on a single value of potential, measured with a regular high-impedance voltmeter with the cathodic protection system operating. The potentials measured with the cathodic protection operating are identified as "on" potential readings. This criterion is very easy to make. However it requires a consideration or elimination of voltage drops in soil between the reference electrode and the structure. Another criterion is based on achieving the same value of structure potential immediately after interrupting the cathodic protection system, and is identified as an "off" potential reading. Another criterion is based on a single value of the structure potential decay, measured from the "off" potential, leaving the cathodic protection system disconnected for several hours or days.
There is no easy and practical method to determine the soil-voltage drop when the "on" potential reading is taken. Therefore, the "off" potential readings, which eliminate the soil voltage drop measured immediately after interrupting the cathodic protection system from the structure, are often used for monitoring corrosion protection levels. However, the "off" potential readings are much more difficult to take than the "on" readings. The interpretation of the "off" potential readings is also much more complex. The "off" potential readings often require use of synchronized current interrupters, fast reacting recorders, oscilloscopes, or wave analyzers. The "off" potential readings after cathodic protection is interrupted can be adversely affected by long-cell currents in the pipeline caused by currents flowing between the more polarized structure sections in the proximity of the rectifiers and less polarized sections, typically in the middle between the rectifiers. Also, the "off" potential readings are often adversely affected by inductive or capacitive voltage spikes, caused by cathodic protection interruption. If the "off" potential reading is taken some time after the spike, some of the polarization is lost and the reading could be therefore invalid.
Meeting the "off" potential criterion often requires that more cathodic protection current be applied than the "on" potential criterion, resulting in possible overprotecting the structure, faster deterioration of the coating, and a higher probability of hydrogen evolution and structure steel embrittlement. The "off" potential measurements are not valid in areas where substantial uninterruptable direct currents are flowing through the soil into or from the structure, polarizing the structure. Such conditions exist, for instance in stray current areas, where the structure is affected by stray currents from electric railroads, from cathodic protection systems on foreign structures, and in areas with telluric (earth) currents naturally induced by fluctuations in the earth's magnetic field. Also, the "off" potential readings cannot be used on structures with distributed galvanic anodes directly connected to the structure.
To eliminate some of the disadvantages of the "off" potential readings on the structure, different cathodic protection test probes and coupon/access tube assemblies have been proposed. A coupon is a metal electrode which simulates a coating holiday on the structure. Coupons are made from the same or similar metal as the structure, and are electrically connected to the structure to receive cathodic protection. The main disadvantages of such prior-art systems can be best demonstrated on cathodically protected and coated pipelines.
Cathodic protection probes with cylindrical coupons, now commercially available, have been described in the above abandoned patent applications and in Material Performance, published by National Association of Corrosion Engineers, Houston, Tex., June 1996, pp. 21-24. The probes consist of a short steel pipe section as a coupon, a plastic tube filled with conductive backfill functioning as an electrolytic "salt bridge", and a porous ceramic plug glued to the end of the plastic tube, representing a potential sensing area. The coupon of the probe simulates a coating holiday on the pipeline. The coupon is electrically connected with insulated wires to the pipeline in a test station and therefore it receives cathodic protection. Cathodic protection probes with cylindrical coupons are easy to manufacture and install. However, they receive the protective current from all sides while the coated pipeline limits the space for the current flow into the coating holiday. The electric field in soil at the cylindrical coupon is therefore different than at the coating holiday. Because the cylindrical coupon receives more current per square area than the coating holiday, the protection level on the coupon could be higher than in the coating holiday.
Measurements of the "on" and "off" potentials in stray current areas and in electric fields around the protected pipelines without any substantial voltage drop in soil require that the distance between the potential sensing area, represented by a porous material surface in contact with soil, and the coupon be very small. This is difficult to accomplish on a probe with a cylindrical coupon because the distance between the center of the sensing area and the coupon is given by the diameter of the pipe section (coupon) and by an application of a heat-shrink tubing over the lower part of the coupon. The heat-shrink tubing is necessary because it eliminates the effect of higher corrosion protection levels at the end of the pipe section close to the sensing area. A very small sensing area in the side of the pipe section substantially decreases the distance between the center of the sensing area and the coupon. However, it provides a small, poor, and unreliable electric contact with soil.
Several different proposals have been made to eliminate the voltage drops in soil when measuring the pipeline potentials against a reference electrode. For instance, U.S. Pat. No. 4,080,565 (1978) to Josef Polak at al., U.S. Pat. No. 4,351,703 (1982) to Joseph D. Winslow, U.S. Pat. No. 4,409,080 (1983) to Carlton M. Slough, U.S. Pat. No. 4,489,277 (1984) and U.S. Pat. No. 4,639,677 (1987) both to Alvin D. Goolsby, U.S. Pat. No. 5,144,247 (1992) to Robert M. Speck, U.S. Pat. No. 5,216,370 (1993) to James B. Bushman, and U.S. Pat. No. 5,469,048 (1995) to Charles W. Donohue propose using coupons or other methods to eliminate the voltage drop in soil for cathodic protection testing.
The Polak et al. patent shows a probe comprising a reference electrode, permanently imbedded inside the probe, and several coupons. This probe suffers from the following disadvantages:
a. The potential readings on the coupons must be taken as "off" readings. The "off" readings should be taken immediately after disconnecting the coupon from the pipeline to eliminate coupon potential decay. This often requires use of fast recorders powered by a battery instead of regular high impedance voltmeters. The "off" potential testing is also more complex and time consuming than the "on" testing with regular voltmeters when no coupon disconnection is required. PA1 b. The probe does not eliminate soil voltage drops from the "off" readings in areas with stray currents which often cannot be interrupted. PA1 c. The semi-permanent reference electrode is built into the probe. This reference electrode cannot be calibrated or monitored for contamination and cannot be replaced without digging out the probe. PA1 a. Testing requires complex circuitry and complicated procedures and interpretation. PA1 b. The probe is not permanently buried in soil. To stabilize the working electrode potential requires time, usually hours. This makes the testing time consuming and costly. PA1 a. The space between the coupon, test station access tube, reference electrode access tube and pipeline is very small and will result in shielding the coupon from the cathodic protection current. This is especially so if the distance between the coupon and the reference electrode access tube were one inch (claim 9), I.e., the top of the coupon sensed by the reference electrode would be substantially shielded from the protective current flowing from the soil into the coupon and the coupon potential will be lower than a similar coating holiday on the pipeline. PA1 b. FIG. 3 shows that the apparatus performs poorly when monitoring the "on" coupon potential readings. The coupon "on" potential with the reference electrode at the earth surface was 1140 mV, the coupon "on" potential with the reference electrode in the reference electrode access tube at the coupon was 1123 mV, and the coupon "off" potential was 950 mV. This indicates that the apparatus eliminated, from the total voltage drop in soil of 190 mV (1140-950 mV), only 17 mV (1140-1123 mV). The effectiveness of this apparatus in eliminating the voltage drop in soil when the "on" potential readings were taken with the reference electrode in the access tube was therefore only 8.9 percent. PA1 c. The distance between the coupon and the reference electrode access tube is not fixed. During installation and after soil settlement, this distance could increase or decrease and change the performance of the apparatus. Also, because the reference electrode access tube is not filled with a conductive backfill, the diffusion rate of oxygen to the top of the coupon will be much higher than to the protected pipeline, resulting in different potentials on the coupon and on the pipeline. PA1 a. to provide an improved, more reliable, and easier-to-use cathodic protection probe; PA1 b. to provide cathodic protection test probe which can be used optionally for the "off" potential readings, but preferably for the more easy-to-do and simple to interpret "on" potential readings without invalidating the readings with any substantial voltage drops in soil; PA1 c. to provide a cathodic protection test probe which represents more closely a typical large coating holiday on the pipeline than other geometries; PA1 d. to provide a cathodic protection test probe with large exposed area of the porous sensing strip to ensure permanent and low resistance contact with soil; PA1 e. to provide a cathodic protection test probe where the coupon is not shielded from the protection current flow by a reference electrode or by access tubes to ensure that the coupon will properly represent a coating holiday of similar size and geometry; PA1 f. to provide a cathodic protection test probe in which the pipeline does not shield the coupon or part of the coupon from the protection current flow; PA1 g. to provide a cathodic protection test probe in which a permanent and low resistance contact with soil is ensured; PA1 h. to provide a cathodic protection test probe in which the distance between the coupon and the porous sensing area is permanently fixed and will not be affected by soil settlement. PA1 i. to provide a cathodic protection test probe in which the reading on the probe represents the least protected area of the coupon; and PA1 j. to provide a cathodic protection test probe which can be manufactured and assembled in a factory, thus ensuring high quality of the product.
Winslow, Jr. recommends installing metallic specimens adjacent to structure. This method could be useful for corrosion rate measurement and determination of cathodic protection current density, but it is not useful for determination of the potentials indicating the level of cathodic protection on the pipeline.
Slough proposes to install a coulomb meter (charge measurer) between the buried structure and a buried small target electrode. This system monitors current flow that relates to a buried structure which is under cathodic protection. It cannot be used to monitor pipeline or coupon potentials to determine the cathodic protection levels.
The Goolsby patents recommend installation of a semi-permanent reference electrode (half cell) on a submerged platform at the metallic plate of a monitoring section. Monitoring for contamination and calibration of the semi-permanent reference electrode is time consuming and costly. It can be done only by use of divers. No means are provided to monitor the stability of the reference electrode to ensure that the potential readings are correct.
The Speck patent describes a cathodic protection monitoring probe having a reference electrode, working and auxiliary electrodes. An electrical contact with ground is provided through a porous ceramic plug at the end of the probe. The recommended method and apparatus suffers from the following disadvantages:
Bushman describes a cathodic protection monitoring system which measures the polarized potential between a reference electrode and a coupon subsequent to decoupling the coupon from the protected structure ("off" potential reading). To eliminate inductive or capacitive voltage spikes caused by the decoupling, Bushman suggests using a complex circuit to determine the potential reading only after the potential has achieved a relatively steady state value.
Donohue, in his preferred embodiment (FIG. 2) shows a cathodic protection measurement apparatus comprising a coupon fixed to the access tube of the test station, an electrode access tube for the reference electrode, electric switch to take "off" potential coupon potential readings, and necessary wiring. Donohue claims that his apparatus will substantially remove any voltage drop in soil if the reference electrode is sufficiently close (one inch) to the coupon. Donohue's system has the following problems and disadvantages: